No, if anyone showed that the observable universe is expanding, it was Römer, when he showed that the speed of light was finite. That’s a completely different observation from what Hubble found.
Is that really you Chronos? Your posts in this thread have grown increasingly (and uncharacteristically) bizarre.
Yes, it’s really me. What’s so bizarre about what I wrote? The fact that the cosmological horizon is receding at c (a natural consequence of the Universe having finite age and c being finite) is completely unrelated to the fact that the scale factor of the Universe is increasing with time (a fact first observed by Edwin Hubble).
Yes I certainly understood what you meant Chronos.
In FLRW cosmology the existance of a particle horizon is infact a little more complicated to derive than merely noting that the speed of light is finite. The believed radius of the observable universe defined as the co-moving radius of the particle horizon is ~46 billion light years.
Whilst the finite speed of nlight plays it’s role, whether or not a particle horizon exists in a FLRW universe is actually dependent on whether or not the intergral of 1/a(t) wrt to t (a(t) being the scale factor and t being the proper time of a comoving observer) converges or not. The dynamics of a FLRW universe are such that there will be a particle horizon in all cases, though in the certain cases where the universe as a whole has a finite volume the particle horizon will meet itself at some point (at the earliest when the universe starts to contract or at the latest at the big crunch).
btw it’s worth mentioning that if the universe did undergo an inflationary phase then the observable universe as defined by the particle horizon would be vastly larger than that predicted by FLRW comsology. However as the inflationatry phase occured long before the surface of last scattering actually using this fact to do some observin’ would need a way of penetrating all the way back to the inflationary era.
Sorry for the newbie question but… if astronomical objects are becoming further apart over time, doesn’t that also imply that said objects are also growing larger? Solid objects are mostly space after all.
While I’m at it, what’s the percentage growth rate of the universe per year?
Though your posts have been rather terse, I think we have established that you differentiate between ‘the observable universe’ and ‘the universe as a whole’, in the following way:
- The observable universe is that which is inside our light cone
- The universe as a whole is that which Hubble showed in the 1930’s to be expanding (see your previous posts).
This seems wildly contradictory to me – obviously Hubble’s observations can apply only to that which is inside his light cone. And this is what I have said earlier – one can only observe the observable universe, and that includes Hubble. Outside of the observable universe, we (and Hubble) have no idea about expansion. And I have given examples repeatedly (such as the string theory landscape, which you have ignored), of concrete, mainstream physics cases for just such a possibility.
The objects don’t grow larger, because the bits that they are made of attract one-another. Suppose you have a spring. If space expands, the spring will start to get stretched, but it will quickly go back to its old shape (in fact far, far, far quicker than the rate of expansion of space). Fast enough that you will never notice it getting stretched.
Well hold on, what Chronos meant is that metric expansion and the expansion of the particle horizon are two distinct phenomena (though as I said a couple of posts above the existance of and size of a particle horizon is directly dependent on the rate of metric expansion). That was in response to a poster who had clearly confused the two.
For example let’s say metric expansion were to suddenly stop, (i.e. the scale factor becomes a constant function at some time, which is admittedly impossible under the dynamics of the big bang model), the particle expansion would continue to expand (and it would expand ‘at the speed of light’).
Now on the seperate issue of the observable universe vs the ‘universe as a whole’, all we’re doing is extending what we know about the portion of the universe we can see in to possible regions that we can’t see simply by applying the cosmological principle to the universe as a whole.
Remember if we look to the edge of the observable universe in one direction and then to the edge in the other direction we will see two points that do not lie in each other’s observable universes. Yet what we see in both directions is pretty much the same*, strongly suggesting at the least that the homogenity and isotropy that we see exists on a sclae much larger than the observable universe.
*infact in some ways my point is invalid as it is too much the same even under the FLRW model, which actually suggests that the two points lie each other’s observable universe. This is where inflation comes in, but I think we should deal with the difference in the predictions of FLRW cosmology and inflationary comsology seperately
The expansion is about 100 km/s/Mpc.
The diameter of the universe is roughly 28Gpc
Therefore the diameter grows by about 3*10^-6 MPc / year
So that means the universe’s diameter grows by about 0.00000001% per year.
It may be we were confused about the line of argument. I was continuing what went back to:
Chronos :
If you define the Universe as “all that exists”, then “outside the Universe” is just as much a contradiction in terms.
Me:
Not when the context is (as provided by the OP) an expanding universe (ie the observable one). But I’m just being pokey.
Chronos :
The fact that the observable universe is expanding is completely unrelated to the fact that the Universe as a whole is expanding. And from the OP’s phrasing, I think it’s clear that he means the Universe as a whole, since what’s beyond the observable universe is not, in any sense, “nothing”: There’s plenty more universe out there.
No matter whether you are talking about metric expansion or the particle horizon, if you try to do science and observe expansion, in both cases we are talking about the observable universe. By definition. In either case, since the OP mentioned the universe which has been observed to be expanding, he must be referring to the observable universe. You seem to disagree with me about this, but I still don’t understand why.
Okay maybe did not remember correctly how this conversation has been going!
However I think it’s clear that what Chronos was doping was trying to delineate the metric expansion and the expansion of the particle horizon. It is easy to extend our model of the universe beyond the particle horizon, all we do is assume that the regions outside of the observable universe are pretty much the same as the region inside the observable universe (though we could bring in infaltion which I believe makes it more difficult to make this kind of assumption).
What people have a problem with is though is that once we have our model of the whole universe by extending what we know beyond the universe,it is still expanding via metric expansion. This is the kind of expansion people are tlaking about when they say “it’s not expanding into anything”. There’s no sensible extension that you can make to the model in order to get the expansion to be the movement of some sort of surface/hypersurface (unless you perfom the redundant exercise of embedding space in some higher dimensional space).
Actually I say that, thoguh of course metric expansion is the ‘movement’ of the hypersurface representing 3-space along the spacetime manifold in the ‘time direction’. That said all that really means is that an expanding universe expands with time, a fairly trivial statement.
OK. My point is that beyond the observable universe (and beyond where we can reasonably extrapolate from data), the universe may or may not be expanding. It may or may not even be of finite size. In may be a local expansion in a much larger landscape. ‘Outside the universe’ could be referring to that landscape. It could be referring to the ‘bulk’. It could be referring to many things that are not ‘nothing’. And even if the universe were of a finite size, it may not make sense to refer to its ‘outside’ as ‘nothing’, because its ‘outside’ could just as well be thought of as a boundary between it and infinite other universes that realize all possibilities, leaving no room for ‘nothing’.
Yep, I think it’s fair point to say our knowledge of the universe is imperfect.
Though I think it’s also fair to say when talking about the metric expansion in FLRW cosmology it makes no sense to talk about the universe expanding in to ‘anything’. And with ‘nothing’ arguably being the opposite of ‘anything’, that’s what leads some people say the universe is expanding in to nothing.
Fair enough, but let it be known there are other cosmologies beyond FLRW, such as Brane cosmology, for which such language is indeed reasonable. And even in FLRW, is it not common to talk of an infinite number of Hubble volumes, expanding, perhaps into each other?
Hehe this thread is why big bang csomology gets so confusing as we’ve introduced yet another type of expansion:D We only have to talk about the cosmological event horizon now and we have pretty much the full set.
The Hubble sphere exists in sensible expanding universes of sufficient volume in FLRW cosmology and represents the sphere of co-moving observers with recessional velocity of c relative to our reference observer. Depending on the exact dynamics of the model it may expand faster or slower than the particle horizion.
Whether or not Hubble volumes of different reference observers will expand in to each other or not given enough time again depends on the dynamics of the universe, but again the expansion of the Hubble sphere, the particle horizon or for that matter the cosmological event horizion should be seen as distinct from, but dependent on, the metric expansion of space.
Quoth iamnotbatman:
Ah, I see the objection now. I was taking the assumption of the Copernican Principle as implicit. Of course, one need not assume the Copernican Principle, and suppose that the space we can observe is significantly different in some way than the space we cannot. But I think that’s a needless complication to the models.
Might the creation and destruction of virtual particles require the expansion of space at the point where they are created? Or perhaps the expansion of space is accompanied by the creation and destruction of virtual particles and thus fuels interactions that drive change?
Hmm… that’s something I haven’t thought about. Sounds kinda reasonable.